The present invention relates to a method of allowing the control of the information transfer, in a data network with closed user groups, between two or more categories of users of the data network.
Different types of data networks are used for information transfer. The structure of the networks varies and is adapted to the field of application. An example of such a network will therefore be described in the following.
For information transfer between mobile units, e.g. vehicles, the information is transferred from terminals in the vehicles via a radio communication link to stationary base stations. These are in turn connected, via permanent circuits, to processor controlled exchanges which process and control the information flow. The exchanges are connected to each other and with possible stationary terminals via a permanent circuit network. Normally the exchanges also are connected to a central control station for the network, which monitors and administrates the operation of the exchanges.
In principle, if no special precautions are taken, all of the users connected to the network can reach all other users. When a network can be used by private persons as well as different companies, police, rescue services etc, this might be unsuitable for various reasons.
This problem has been solved by a system with so-called Closed User Groups (CUG). By identifying a number of users as belonging to a certain closed user group, the exchanges of the network can be controlled in such a way that a user can only have communication with the other users in the group. As every user may be allowed to be part of several closed user groups it is however possible to communicate between the groups. The system is well-known and standardized e.g. by means of international telecommunication standards such as CCITT X.25.
However, a problem arises with this system in the case where different categories of users within the closed user groups should be able to communicate with a category of users which is not part of the closed user groups. An example of such a category of users is public services, i.e. services which should be available to all network users. Electronic mail, which is a service for transfer of messages, is such a public service.
As these categories of users or services must be localized outside of the existing closed user groups, the system effectively prevents them from being reached by the group members. In circuit-switched data network with the possibility of virtual circuit-switched connections between different users, the problem may be solved by letting one user be open only for incoming or only for outgoing traffic. Users of, for example, a public service are then only given the possibility of outgoing traffic to the special closed user group which is attached to the service in question. As the network uses virtual circuit-switched connections, a complete dialogue may be made as soon as the connection has been established.
In networks lacking this type of connection which is the case in networks intended for transmission of datagrams, or packets, this possibility does not exist as the connection only operates in one direction.